1. Field of the Invention
The present invention relates to an antenna device and a communication terminal device that are preferably used for a communication system for the HF band and the UHF band.
2. Description of the Related Art
Among devices provided for electronic devices such as mobile phones and that perform HF band communication such as near field communication (NFC), an RFIC and a matching element are generally mounted on a circuit board, and an antenna is attached to an inner surface of an enclosure of an electronic device. The RFIC and the antenna are electrically connected to each other through spring pins or the like.
Meanwhile, wireless communication terminals such as recent mobile phones have been made increasingly thinner. To compensate for insufficient strength caused by such reduction in thickness, there are more cases than before where an enclosure is “metallized” such as a case where an enclosure is plated with magnesium.
However, in the case where an enclosure of a terminal is “metallized”, there arises a problem in which communication with a counterpart antenna is prevented, because an electromagnetic field around an antenna incorporated in the terminal is shielded by a metal.
Hence, as disclosed in Japanese Patent No. 4993045, there is proposed an antenna device having a configuration in which a metal plate having a larger area than an antenna coil is located in proximity to the antenna coil (to induce magnetic field coupling) so as to use the metal plate as a radiator.
The antenna configuration described in Japanese Patent No. 4993045 enables communication with a counterpart antenna, despite the antenna being covered with a metal. However, in a case where the metal plate is provided with a slit or an opening portion, consideration of decreased mechanical strength is required, and thus the number of man-hours required for production is increased. In addition, particularly in a case where a metal enclosure is provided with a slit or an opening, there arises a restriction on enclosure design. Further, since a portion in the vicinity of the slit or the opening portion cannot be connected to the ground of a circuit, portions of the metal plate might have a variation in potential. This causes a problem in which a field shield effect due to use of the metal plate is deteriorated, and a concern in which a first conductor surface and a second conductor surface might interfere with another radio frequency circuit.